Method for reproducing plating solution

ABSTRACT

A plating solution containing a leveler is brought into contact with silica particles with an average particle diameter of 500 μm or less to remove impurities from the plating solution.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Japanese PatentApplication No. 2019-220573, filed on Dec. 5, 2019, the entiredisclosure of which as is incorporated by reference herein in itsentirety.

BACKGROUND

The present invention relates to a method for reproducing a platingsolution.

In integrated circuit (IC) packages, for example, solder paste andsolder balls have widely been used for connection with ICs. In recentyears, electrode areas have been microminiaturized, and thus it has beendifficult for the conventional solder paste and the like to cope withthe situation.

Given these circumstances, as a measure therefor, a method is proposedwhich uses an electrolytic tin plating solution or an electrolytic tinalloy plating solution for formation of bumps of semiconductor chips.More specifically, a plating solution is proposed which contains asoluble salt containing a stannous salt, and an acid selected from anorganic acid and an inorganic acid or a salt thereof, two kinds ofsurfactants, namely, an amine-based surfactant and a nonionicsurfactant, and an additive such as indole, for example. It is shownthat use of such a plating solution achieves height uniformity of solderbumps over a wide current density range, and prevents or reducesoccurrence of voids during formation of the bumps (see, e.g., JapaneseUnexamined Patent Publication No. 2018-162512).

BRIEF SUMMARY

In the plating solution for bump formation, an additive such as indoleis used as in Japanese Unexamined Patent Publication No. 2018-162521because a plated coating is required to be uniform and to have a flatsurface. Unfortunately, when electrolytic treatment is performed on theplating solution, this additive changes to form impurities (awater-insoluble substance as a byproduct accompanying decomposition ofthe additive), and consequently, it is disadvantageously impossible toprevent or reduce occurrence of voids within the plated coating afterreflow.

In view of the foregoing problems, it is therefore an object of thepresent invention to provide a method for reproducing a platingsolution, the method including removing impurities from a platingsolution to prevent or reduce occurrence of voids within a platedcoating after reflow.

To achieve the above object, a method for reproducing a plating solutionaccording to the present invention includes bringing a plating solutioncontaining a leveler into contact with silica particles with an averageparticle diameter of 500 μm or less to remove impurities from theplating solution.

The present invention can remove impurities from a plating solution andcan thus prevent or reduce occurrence of voids within a plated coatingafter reflow.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic diagram for illustrating a method for reproducinga plating solution of the present invention.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

The following describes a method for reproducing a plating solution ofthe present invention.

Plating Solution to be Treated

A plating solution to which the method of reproduction of the presentinvention is applied is not limited to a particular plating liquid.Examples thereof include an electrolytic tin plating solution and anelectrolytic tin alloy plating solution for use in formation ofconnection bumps.

The electrolytic tin plating solution or the electrolytic tin alloyplating solution can contain a tin salt as a compound as a tin ionsupply source. In one preferred embodiment, a stannous salt (tinsalt(II)) and a stannic salt (tin salt(IV)) are used.

The stannous salt (tin salt(II)) is not limited to a particular stannoussalt. Examples thereof include tin(II) alkane sulfonates such as tin(II)methane sulfonate, tin(II) organic sulfonates such as tin(II) alkanolsulfonates such as tin(II) isethionate, tin(II) sulfate, tin(II)borofluoride, tin(II) chloride, tin(II) bromide, tin(II) iodide, tin(II)oxide, tin(II) phosphate, tin(II) pyrophosphate, tin(II) acetate,tin(II) citrate, tin (II) gluconate, tin(II) tartrate, tin(II) lactate,tin(II) succinate, tin(II) sulfamate, tin(II) formate, and tin(II)silicofluoride.

The stannic salt (tin salt(IV)) is not limited to a particular stannicsalt. Examples thereof include sodium stannate and potassium stannate.In one preferred embodiment, tin(II) alkane sulfonates such as tin(II)methane sulfonate and tin(II) organic sulfonates such as tin(II) alkanolsulfonates such as tin(II) isethionate are used.

The concentration of the tin salt (the concentration as Sn²⁺) ispreferably 5 g/L or more, more preferably 10 g/L or more in order toreduce the occurrence of burning and scorching in the film. Theconcentration of the tin salt is preferably 120 g/L or less, morepreferably 90 g/L or less in order to improve the stability of a platingbath and reduce the occurrence of precipitates. Being such aconcentration is advantageous also in view of costs.

As the tin salt, a low-concentration lead tin salt with a lead (Pb)concentration of 1.0 ppm or less can also be used. Use of thelow-concentration lead tin salt can achieve low-concentration lead.

The electrolytic tin alloy plating solution can contain a silver salt asa compound as a silver ion supply source. This silver salt is notlimited to a particular silver salt. Examples thereof include silverorganic sulfonates, silver sulfate, silver borofluoride, silverchloride, silver bromide, silver iodide, silver oxide, silver phosphate,silver pyrophosphate, silver acetate, silver citrate, silver gluconate,silver tartrate, silver lactate, silver succinate, silver sulfamate,silver formate, and silver silicofluoride. Among these, in oneparticularly preferred embodiment, silver organic sulfonates are used.

The concentration of the compound as the silver ion supply source (theconcentration as Ag⁺) is preferably 10 mg/L or more, more preferably 20mg/L or more in order to easily control the plating solution. Theconcentration of the compound as the silver ion supply source is 1,000mg/L or less, more preferably 500 mg/L or less in view of costs.

The electrolytic tin alloy plating solution may further contain acompound as a copper (Cu) ion supply source. Addition of the compound asthe copper ion supply source can form a film of a Sn—Ag—Cu ternaryalloy.

As the copper ion supply source, a copper salt can be used and thiscopper salt is not limited to a particular copper salt. Examples thereofinclude copper organic sulfonates, copper sulfate, copper borofluoride,copper chloride, copper bromide, copper iodide, copper oxide, copperphosphate, copper pyrophosphate, copper acetate, copper citrate, coppergluconate, copper tartrate, copper lactate, copper succinate, coppersulfamate, copper formate, and copper silicofluoride. Among these, inone particularly preferred embodiment, copper organic sulfonates areused.

The concentration of the compound as the copper ion supply source (theconcentration as Cu⁺) is preferably 10 mg/L or more, more preferably 50mg/L or more in order to easily control the plating solution. Theconcentration of the compound as the copper ion supply source ispreferably 5,000 mg/L or less, more preferably 2,000 mg/L or less inview of bath stability.

The electrolytic tin plating solution or the electrolytic tin alloyplating solution contains a leveler in order to improve the uniformityof a plated coating and the flatness of the surface shape thereof. Asthis leveler, a nitrogen-containing aromatic compound is used, forexample. Examples of this nitrogen-containing aromatic compound includeindole, pyridine, pyrazine, pyrimidine, pyridazine, triazine, tetrazine,acridine, quinoline, isoquinoline, quinoxaline, quinazoline, cinnoline,indazole, imidazole, benzimidazole, isoindole, benzothiazole, phenazine,iminostilbene, quinaldine, purine, 1,10-phenanthroline, carbazole,acrinol, benzotriazole, benzoxazole, and derivatives thereof.

The concentration of the nitrogen-containing aromatic compound ispreferably 0.001 g/L or more, more preferably 0.01 g/L or more in orderto improve the flatness of the surface shape of the film. Theconcentration of the nitrogen-containing aromatic compound is preferably20 g/L or less, more preferably 10 g/L or less in view of costs.

The electrolytic tin plating solution or the electrolytic tin alloyplating solution may contain any of an inorganic acid, an organic acid,and a water-soluble salt thereof. Addition of the acid or water-solublesalt thereof allows the pH of the surface of an object to be plated anda tin surface or Sn—Ag alloy surface serving as the plated coating to bemade uniform, thus achieving uniform surface electric potential. Thiscan reduce co-deposition of lead.

The acid or water-soluble salt thereof is not limited to particular one.Examples thereof include sulfuric acid, hydrochloric acid, nitric acid,phosphoric acid, sulfamic acid, organic sulfonic acids (alkane sulfonicacids such as methane sulfonic acid and alkanol sulfonic acids such asisethionic acid), and carboxylic acids (aromatic carboxylic acids,aliphatic saturated carboxylic acids, and amino carboxylic acids).Neutralization salts of these water-soluble salts can also be used asneeded. Among these, in one preferred embodiment, methane sulfonic acid,which is easy to handle, is used.

The concentration of the acid or water-soluble salt thereof ispreferably 35 g/L or more, more preferably 50 g/L or more in order toimprove the stability of the plating solution and reduce occurrence ofprecipitates. Such a concentration is advantageous also in view of leadprecipitation potential. The concentration of the acid or water-solublesalt thereof is preferably 500 g/L or less, more preferably 300 g/L orless, even more preferably 200 g/L or less in view of costs.

The electrolytic tin plating solution or the electrolytic tin alloyplating solution may contain a surfactant. As the surfactant, one ormore selected from an anionic surfactant, a cationic surfactant, and anonionic surfactant can be used. Among these, in one preferredembodiment, the nonionic surfactant is used, and in one more preferredembodiment, an alkylene oxide-based one is used. Addition of thesurfactant allows the current density of the object to be plated and atin crystal surface serving as the plated coating to be made uniform,thus maintaining uniform precipitation potential on the surface. Thiscan reduce co-deposition of lead.

The alkylene oxide-based surfactant is not limited to a particularsurfactant. Examples thereof include polyoxyethylene alkyl ethers,polyoxyethylene alkylphenyl ethers, polyoxyethylene alkyl amines,polyoxyethylene alkyl amides, polyoxyethylene aliphatic esters,polyoxyethylene polyhydric alcohol ethers, ethylene oxide-propyleneoxide block copolymerized compounds, ethylene oxide-propylene oxiderandom copolymerized compounds, and propylene oxide polymerizedcompounds. Among these, in one preferred embodiment, polyoxyethylenealkylphenyl ethers are used.

The concentration of the surfactant is preferably 0.05 g/L or more, morepreferably 0.5 g/L or more. Being such a concentration, even whenplating is performed with a high current density in order to reduce aplating time, can reduce occurrence of burning and scorching in partswith a high current density. The concentration of the surfactant ispreferably 100 g/L or less in order to reduce the occurrence of colorunevenness caused by blackening of the plated coating.

The electrolytic tin plating solution or the electrolytic tin alloyplating solution contains an acid or water-soluble salt thereof. Theacid or water-soluble salt thereof is one or more acids or water-solublesalts thereof selected from sulfuric acid, hydrochloric acid, nitricacid, phosphoric acid, sulfamic acid, organic sulfonic acids, andcarboxylic acids or salts thereof.

The electrolytic tin plating solution or the electrolytic tin alloyplating solution can contain an organic solvent, an antioxidant, and achelating agent. The organic solvent is not limited to a particularorganic solvent. Examples thereof include monohydric alcohols such asmethanol and 2-propanol and dihydric alcohol such as ethylene glycol,diethylene glycol, and triethylene glycol. The antioxidant is notlimited to a particular antioxidant. Examples thereof include catechol,hydroquinone, 4-methoxyphenol, and ascorbic acid. The chelating agent isnot limited to a particular chelating agent. Examples thereof includeoxalic acid, succinic acid, malonic acid, glycolic acid, gluconic acid,gluconolactone, glycine, ethylenediamine acetic acid, pyrophosphoricacid, and tripolyphosphoric acid.

When the plated coating is formed using the electrolytic tin platingsolution or the electrolytic tin alloy plating solution, in onepreferred embodiment, the pH of the plating solution is strongly acidic.Temperature when the plated coating is formed is not limited to aparticular temperature. In one preferred embodiment, the temperature is20° C. or more and 40° C. or less. Current density when the platedcoating is formed is preferably 0.1 A/dm² or more, more preferably 0.5A/dm² or more and is preferably 20 A/dm² or less, more preferably 10A/dm² or less.

The electrolytic tin plating solution or the electrolytic tin alloyplating solution can be used for formation of plated bumps ofsemiconductor chips and package substrates, for example. In forming theplated bumps, reflow treatment may be performed after a plated coatingwith a certain size is formed at a certain position. This reflowtreatment is not limited to particular reflow treatment and can beperformed using a normal reflow apparatus.

Method for Reproducing Plating Solution

As described above, when electrolytic treatment is performed using theelectrolytic tin plating solution or the electrolytic tin alloy platingsolution containing the leveler such as indole, there is a problem inthat caused by decomposition of the leveler, the impurities (thewater-insoluble substance as a byproduct accompanying decomposition ofthe leveler, for example) are generated in the plating solution, andconsequently, voids occur within the plated coating after reflow.

Given these circumstances, the inventors of the present invention havestudied the above problem to find out that the plating solutioncontaining the impurities is circulated to be brought into contact withsilica particles (SiO₂), whereby the impurities can be removed from theplating solution.

The following specifically describes the method for reproducing aplating solution of the present invention with reference to theaccompanying drawing. FIG. 1 is a schematic diagram for illustrating themethod for reproducing a plating solution of the present invention.

As illustrated in FIG. 1, the method for reproducing a plating solutionof the present invention circulates a plating solution 2 (that is, theelectrolytic tin plating solution or the electrolytic tin alloy platingsolution containing the leveler such as indole described above)contained in a plating tank 1 by a pump 3 and brings the platingsolution 2 into contact with silica particles 5 contained in an impurityremoving device 4 disposed in a circulating path of the plating solution2 to remove impurities 6 from the plating solution 2.

More specifically, first, the plating solution 2 is circulated to bebrought into contact with the silica particles 5 to cause the levelerwithin the plating solution 2 to adsorb onto the silica particles 5.Next, electrolytic treatment is performed with the plating solution 2 inthe plating tank 1 to form the plated bumps described above, forexample. In this process, as described above, the impurities 6 aregenerated in the plating solution 2 caused by decomposition of theleveler; the plating solution 2 is constantly circulated, whereby asillustrated in FIG. 1, the impurities 6 are brought into contact withthe silica particles 5 and are adsorbed onto the silica particles 5instead of the leveler.

Consequently, the silica particles 5 can remove the impurities 6 fromthe plating solution 2, and consequently, this can prevent or reduceoccurrence of voids within the plated coating after reflow.

As illustrated in FIG. 1, the plating solution 2 from which theimpurities 6 are removed is circulated by the pump 3 to be returnedagain to the plating tank 1.

The impurity removing device 4 is not limited to a particular device.Examples thereof include a bag filter. An opening of this bag filter isnot limited to a particular value. In one preferred embodiment, theopening is 4 μm or less in order to surely prevent an outflow of thesilica particles contained in the filter (that is, filter passing)caused by circulation of the plating solution.

In the present invention, an average particle diameter of the silicaparticles is 500 μm or less in order to surely perform adsorption of theimpurities. When the average particle diameter is larger than 500 μm,the pore size of the silica particles is large, and selectiveadsorptivity for the impurities is lost. That is to say, this is becausewhen the size of the silica particles increases, matter other than theimpurities is also adsorbed and removed, which may make removal of theimpurities difficult (that is, reduce impurity removal performance).

In order to surely prevent the filter passing described above, in onepreferred embodiment, the lower limit value of the average particlediameter of the silica particles is larger than the opening of the bagfilter described above; more specifically, in one preferred embodiment,the lower limit value is 20 μm or more.

The “average particle diameter” referred to in this specificationindicates a 50% particle diameter (D50) and can be measured with, e.g.,a particle size distribution measurement apparatus using the laserDoppler method (manufactured by Nikkiso Co., Ltd., Nanotrac (registeredtrademark) particle size distribution measurement apparatus UPA-EX150).

EXAMPLES

The following describes the invention according to the presentapplication more specifically based on examples and comparativeexamples. The present invention is not limited to the following examplesat all.

Example 1

Formation of Plated Coating First, a base material was subjected toelectrolytic nickel plating (electrolytic nickel plating solution:manufactured by C. Uyemura & Co., Ltd., product name: Thrunic AMT,liquid temperature: 50° C., current density: 1 A/dm², plating time: 10minutes).

Next, mixing was performed so as to contain tin(II) alkane sulfonate asa tin salt of 70 g/L as tin (Sn²⁺), methane sulfonic acid as an organicacid of 100 g/L, polyoxyethylene bisphenol A ether as a surfactant of 50g/L, and indole of 5 g/L, and the mixture was stirred to prepare anelectrolytic tin plating solution of the present example.

Next, this electrolytic tin plating solution was contained in a platingtank. While the electrolytic tin plating solution was circulated using apump, using the electrolytic tin plating solution, a tin plated coatingwas formed on the surface of nickel as the base material with a liquidtemperature of 30° C. and a current density of 4 A/dm².

A bag filter as an impurity removing device (manufactured by Eaton,product name: LCR-113-TO1E-401, opening: 4 μm) was disposed in acirculating path of the plating solution. Silica particles with anaverage particle diameter of 50 μm (manufactured by Evonik Industries,product name: Sipernat 50) in an amount of 1 kg was contained in the bagfilter. A tin plated coating was formed while the electrolytic tinplating solution was circulated using a pump to bring the electrolytictin plating solution into contact with the silica particles.

Void Evaluation

After being reflowed at 260° C., the obtained tin plated coating wasevaluated for the presence or absence of voids with an X-raynondestructive inspection apparatus (manufactured by Nordson Dage,product name: XD7600NT Diamond FP). The X-ray nondestructive inspectionapparatus was set to give a tube voltage of 60 kV and an output of 1.5W. Table 1 lists a result of the foregoing.

Example 2

A tin plated coating was formed, and void evaluation was performed in amanner similar to Example 1 except that the average particle diameter ofthe silica particles was changed to 120 μm (product name: CARPLEV XRmanufactured by Evonik Industries was used). Table 1 lists a result ofthe foregoing.

Example 3

A tin plated coating was formed, and void evaluation was performed in amanner similar to Example 1 except that the average particle diameter ofthe silica particles was changed to 300 μm (product name: Nipsil AQmanufactured by Tosoh Silica Corporation was used). Table 1 lists aresult of the foregoing.

Comparative Example 1

A tin plated coating was formed, and void evaluation was performed in amanner similar to Example 1 except that the bag filter containing thesilica particles was not disposed in the circulating path of the platingsolution, and the plating solution was not brought into contact with thesilica particles. Table 1 lists a result of the foregoing.

Comparative Example 2

A tin plated coating was formed, and void evaluation was performed in amanner similar to Example 1 except that silica particles with an averageparticle diameter of larger than 500 μm (manufactured by Fuji SilysiaChemical Ltd., product name: Fuji silica gel ID40) was used in place ofthe silica particles with an average particle diameter of 50 μm. Table 1lists a result of the foregoing.

TABLE 1 Compar- Compar- ative ative Example 1 Example 2 Example 3Example 1 Example 2 Presence or Present Present Present Absent PresentAbsence of Silica Treatment Average 50 120 300 None >500 ParticleDiameter of Silica Particles (μm) Occurrence Not Not Not OccurredOccurred of Voids Occurred Occurred Occurred

As listed in Table 1, in Examples 1 to 3, the tin plated coating wasformed while the plating solution containing the leveler was broughtinto contact with the silica particles with an average particle diameterof 500 μm or less, and it is thus shown that the impurities in theplating solution were removed by the silica particles, and consequently,no voids occurred within the plated coating after reflow.

On the other hand, in Comparative Example 1, the plating solution wasnot brought into contact with the silica particles, and it is thus shownthat the impurities were not removed from the plating solution, andvoids occurred within the plated coating after reflow.

In Comparative Example 2, the average particle diameter of the silicaparticles was larger than 500 μm, and it is thus shown that theimpurities were not removed from the plating solution, and voidsoccurred within the plated coating after reflow.

The method for reproducing a plating solution of the present inventionis suitably used in plating solutions for use in formation of platedbumps of semiconductor chips and package substrates in particular.

1. A method for reproducing a plating solution comprising: bringing aplating solution containing a leveler into contact with silica particleswith an average particle diameter of 500 μm or less to remove impuritiesfrom the plating solution.
 2. The method of claim 1, wherein the platingsolution is circulated from a plating tank containing the platingsolution and the plating solution from which the impurities are removedby the circulation is returned to the plating tank.
 3. The method ofclaim 1, wherein the silica particles are contained in an impurityremoving device disposed in a circulating path for the plating solution.4. The method of claim 3, wherein the impurity removing device is a bagfilter.
 5. The method of claim 4, wherein an average particle diameterof the silica particles is larger than an opening of the bag filter. 6.The method of claim 1, wherein the plating solution is an electrolytictin plating solution or an electrolytic tin alloy plating solution. 7.The method of claim 1, wherein the leveler is indole.